1. Field of the Invention
This invention relates to a wheel acceleration slip control system which controls the rotation of drive wheels to increase the frictional force between the drive wheels and the road surface at the time of acceleration.
2. Background of the Related Art
For obtaining the most suitable braking force without wheel lock at the time of braking, so called anti-skid control system has been popularly used. This system works in such a manner that the wheel speed (rotation speed multiplied by the wheel circumference) during a braking is controlled to stay a little lower than the vehicle running speed ("vehicle speed" hereinafter), resulting in the greatest friction force between the wheel tire and the road surface. The formula below shows the relation among the slip rate percentage (S), the vehicle speed (Vs) and the wheel speed (V). EQU S=[(Vs-V)/Vs].times.100
As shown in FIG. 6, when the slip rate (S) comes close to 10%, the friction force M between the tire and the road surface is the greatest, and the side force F which is the resistance force against a side slip is not so small. For this reason braking and its release are alternatively and properly repeated to control the vehicle speed (V) in order to obtain the slip rate (S) around 10%.
So-called traction control has been proposed for the problems arising during acceleration (Japan patent application No. Sho 59-275532, Japan patent application No. Sho 60-294439 and others). This is intended to control an acceleration slip that prevents sufficient acceleration and that may result in inefficient fuel consumption. This control system employs a feedback control in which an acceleration slip control is started when a drive wheel slip exceeds a predetermined value. Control is achieved by means of engine output control or brake control or even by the both in order to obtain a low slip value.
In the acceleration slip control system described above, slip conditions for which feedback control is started, i.e., the threshold conditions are preferably set to correspond to a higher slip rate or a higher slip speed than a target value at which the feedback control system should ideally be targeted. This is to prevent the system from overreacting to a minor roughness or tiny muddy portion of the road. (This is shown in the Japan patent application No. Sho 59-275532.)
In this feedback system, when the drive wheel's slip exceeds the threshold, both the engine output control and brake control are started to lower the slip rate to a predetermined target value below the threshold. The engine output is controlled by a secondary throttle valve provided in series with a normal main throttle valve in the intake pipe of the internal combustion engine. The main throttle valve is linked to the accelerator pedal and the secondary throttle valve is rotated by a motor controlled by an electronic circuit. The opening of the secondary throttle valve is decreased and/or the brake is applied when the slip is greater than the target and the secondary throttle valve opening is increased and/or the brake is released when the slip is less than the target.
The acceleration feedback control including the brake control is finished when the secondary throttle valve reaches substantially its open-most position. A new feedback control is started when the driving wheel slip exceeds the threshold, which is higher than the feedback control target.